1. Field of the Invention
The present invention relates to an organic solution of a fluorinated copolymer having carboxylic acid groups. More particularly, it relates to a novel organic solution of a fluorinated copolymer having carboxylic acid groups (--COOH) dissolved in a hydrophilic organic solvent with a small amount of water, at high concentration.
2. Description of the Prior Art
Fluorinated polymers are usually durable to organic solvents and accordingly, organic solutions of the fluorinated polymer have not been substantially known. Especially organic solvents which can dissolve fluorinated polymers having many fluorine atoms bonded to each carbon atoms of a main chain skeleton have not been substantially known.
On the other hand, if such solution of the fluorinated polymer is obtained, various applications of the fluorinated polymers will be developed.
It has been noted that copolymers of a fluorinated olefin such as tetrafluoroethylene and a fluorinated monomer having carboxylic acid type group can be used for a cation exchange membrane having excellent oxidation resistance, chlorine resistance, alkali resistance and heat resistance which is used for producing an alkali metal hydroxide and chlorine, and also used for a membrane for a fuel cell and a membrane for dialysis and other uses. If an organic solvent solution of the carboxylic acid type fluorinated polymer can be obtained, the fabrication of the membrane is easily made whereby a membrane having a complicated configuration or a remarkably thin membrane can be obtained, or a membrane can be smoothly obtained by a dipping process, or pin holes of a membrane can be easily repaired, or a surface of a product can be easily coated by the fluorinated polymer. Various advantages are considered by the preparation of the solutions.
In the case of the fluorinated polymers having strong acidic groups having high polarity such as sulfonic acid groups, it has been known that the fluorinated polymer having only specific form such as sulfonic acid, sulfamide or sulfonate can be dissolved into a special organic solvent having high polarity as disclosed in Japanese Patent Publication No. 13,333/1973. Thus, in the case of the fluorinated polymer having carboxylic acid groups, the polymers can not be dissolved into the organic solvents for dissolving sulfonic acid type fluorinated polymer because of carboxylic acid group of the polymer.
The organic solvent solutions of carboxylic acid type fluorinated polymer have been found as disclosed in Japanese Unexamined Patent Publication No. 107949/1979 as the solutions of fluorinated polymer having carboxylic acid groups as --COOQ (Q: alkali metal atom) in an organic solvent having high polarity such as alcohols and glycols or Japanese Patent application No. 56912/1979 as the solutions of fluorinated polymer having pendant carboxylic ester groups in an organic fluorinated solvent such as trichlorotrifluoroethane and benzotrifluoride.
According to the studies, it is difficult to increase a concentration of the fluorinated polymer in such organic solvent solutions. Only the solution having a concentration of up to 5 wt. % has been obtained. It is preferable to increase the concentration of the fluorinated polymer in a fabrication of a membrane from the solution.